1. Field of the Invention
The present invention relates to a toner transfer roller used in an image forming device such as a copying machine, a printer, a plotter, a facsimile machine and the like.
2. Description of Related Art
Japanese Laid-Open Patent Application (Kokai) No. HEI-6-155798 describes an image forming device including a modulating electrode member. The modulating electrode member has a plurality of tiny pores (hereinafter referred to as "apertures") therein, and toner passage through the apertures is controlled in accordance with a drive signal applied to respective control electrodes connected to the apertures. The toner particles selectively pass through the apertures and form an image on an image recording medium.
More specifically, as shown in FIG. 1, this type of image forming device 100 includes a modulating electrode member 101, a toner transfer roller 114, a back electrode roller 122, a frame 111 and a circuit 108. The toner transfer roller 114 is made of metal and is rotatably disposed in confrontation with an image recording medium 120 with the modulating electrode member 101 intervened therebetween. The back electrode roller 122 is rotatably disposed in a position opposite the toner transfer roller 114 with respect to the recording medium 120. As shown in FIG. 2, the modulating electrode member 101 includes a 25 .mu.m thick insulation sheet 102 made of polyimide. The insulation sheet 102 is formed with a plurality of apertures 106, each aperture 106 having a diameter of 100 .mu.m. The apertures 106 are aligned in a longitudinal direction of the modulating electrode member 101.
Control electrodes 104 are formed on a surface of the insulation sheet 102 facing the image recording medium 120. Each control electrode 104 surrounds a respective one of the apertures 106 and is formed to a thickness of 1 .mu.m and a width of 20 .mu.m from cooper foil. The circuit 108 applies control voltages to the control electrodes 104 so that toner particles 116 are selectively pulled through the apertures 106 toward the image recording medium 120. The control signal corresponds to an image signal.
A toner regulation blade 118 is provided for regulating a toner layer on a surface of the toner transfer roller 114 in order to provide a uniform toner layer of uniformly charged toner particles 116.
However, the image forming device 100 described above has some drawbacks. When the modulating electrode member 101 is assembled askew or obliquely fixed to the frame 111, the toner transfer roller 114 may press against the modulating electrode member 101 with poor uniformity along its length. This causes density variation on a recorded image because more toner particles 116 flow through the apertures 106 with the toner transfer roller 114 strongly presses than through the apertures 106 where the toner transfer roller 114 presses with less force.
The amount of the toner particles 116 that flow through each aperture 106 toward the image recording medium 120 also varies depending on the thickness of the toner layer supplied to the apertures 106. More specifically, if the apertures 106 are supplied with an insufficient amount of toner particles 116, then too few toner particles 116 will flow therethrough resulting in toner clinging to the image recording medium 120 in an in sufficient amount. On the other hand, if the apertures 106 are supplied with an excessive amount of toner particles 116, then excessive toner particles 116 will pass therethrough resulting in excessive toner clinging to the image forming medium 120. Also, when excessive toner particles are deposited on the toner transfer roller 114, the toner may enter the apertures 106 when not desired, potentially clogging the apertures 106 or causing fogging on the image forming medium 120.
A variation in the thickness of toner layer with respect to a rotational direction of the toner transfer roller 114 results in variation in density of the recorded image with respect to a sheet feeding direction of the image recording medium 120. On the other hand, variation in thickness of toner layer with respect to a longitudinal direction of the toner transfer roller 114 results in a variation in density of the recorded image with respect to a direction perpendicular to the sheet feeding direction.
A non-uniform pressing force of the blade 118 against the transfer roller 114 is a main cause of variation in thickness of the toner layer. There are various factors which can cause a non-uniform pressing force. For example, the portion of the blade 118 that contacts the transfer roller 114 may not extend with complete straightness in the longitudinal direction along the transfer roller 114. A rounded tip portion R of the blade 118 may have a shape variation. The blade 118 may provide surface irregularities, and undulations. Further, the inaccurate assembly of the blade may cause the variation in toner thickness.